Gas diffusing electrode body, method of manufacturing the same and electrochemical device

ABSTRACT

A gas diffusing electrode, an electrochemical device, such as fuel cell, employing same and methods of manufacturing same are provided. The gas diffusing electrode at least includes layers made of at least electro-conductive carbon powder or granule and sputtered platinum layers made of platinum laid alternately to form a multilayer structure. The electrochemical device can be down-sized while maintaining a relatively high power generating capacity.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application claims priority to Japanese Patent Document No.P2001-064814 filed on Mar. 8, 2001, the disclosure of which is hereinincorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a gas diffusing electrode body that canbe used, for example, for manufacturing a fuel cell, to a method ofmanufacturing such an electrode body and also to an electrochemicaldevice.

In recent years, there has been an ever-increasing strong demand foralternative clean energy sources that can replace fossil fuel includingpetroleum. Hydrogen and hydrogen gas fuel are attracting attention assuch energy sources.

It is believed that hydrogen is an ideal energy source because itcontains a large amount of chemical energy per unit mass and emitsneither harmful substances nor gas that warms the earth. Hydrogen is aclean, ubiquitous and inexhaustible energy source.

Recently, efforts have been paid to develop fuel cells adapted to drawelectric energy out of hydrogen energy. Expected applications of suchfuel cells include large scale power plants, on-site home powergenerators and power sources of electric automobiles.

A fuel cell is formed by arranging fuel electrodes, which may typicallybe a hydrogen electrode and an oxygen electrode, oppositely with aproton conductor film sandwiched between them. A cell reaction occurs togive rise to electromotive power as hydrogen and oxygen are supplied tothe respective electrodes as fuel. When manufacturing a fuel cell, aproton conductor film and fuel electrodes, which may typically include ahydrogen electrode and an oxygen electrode, are molded separately andsubsequently bonded together.

The fuel electrodes, or gas diffusing electrodes, of a fuel cell, whichmay typically include a hydrogen electrode and an oxygen electrode, aremainly made of electro-conductive carbon particles and have a catalystlayer that carries catalyst metal such as platinum.

Conventionally, a gas diffusing electrode is manufactured either bymolding catalyst particles, which are powdery or granularelectro-conductive carbon particles carrying platinum as catalyst, intoa sheet along with water-repelling resin such as fluorine resin and anion conducting material or by using a step of directly applying thecatalyst and the other ingredients onto a carbon sheet. Japanese PatentApplication Laid-Open Publication No. 5-36418 discloses a gas diffusingelectrode to be used for a solid polymer fuel cell, which ismanufactured by applying powdery or granular carbon carrying platinumonto a carbon sheet along with water-repelling resin and an ionconducting material.

A gas diffusing electrode refers to an electrode having continuous poresthrough which working gas can be diffused. A gas diffusing electrodealso shows an electron conducting property.

When a gas diffusing electrode is used as hydrogen decomposing electrodeof a fuel cell that may be of a solid polymer type, fuel is ionized bythe catalyst of platinum and electrons produced by ionization flowsthrough the powdery or granular electro-conductive carbon, while protons(H⁺s) produced as a result of ionization of hydrogen flows to the ion(proton) conducting film by way of ion conductors. This process requiresgaps allowing gas to pass through, electro-conductive powdery orgranular carbon, ion conductors and a catalyst for ionizing fuel and/oran oxidizing agent. Fuel is ionized by the catalyst which is typicallyplatinum and electrons produced as a result of ionization flows throughthe electro-conductive powdery or granular carbon, while ionizedhydrogen (protons) flows to the ion conducting film by way of the ionconducting material. This process requires gaps allowing gas to passthrough, electro-conductive powdery or granular carbon, ion conductorsand a catalyst for ionizing fuel and/or an oxidizing agent. If fuel ishydrogen, a reaction ofH₂→2H⁺⁺2^(e−)

takes place in the gas diffusing electrode (catalyst layer) of the fuelcell, while a reaction ofO₂+4H⁺⁺4^(e−)→2H₂O

occurs in the oxygen electrode to produce water.

There is an increasing demand for fuel cells of the type underconsideration that can generate a large amount of power per unit volume.However, the use of a large gas diffusing electrode is required toincrease the rate of power generation. On the other hand, there is anurgent demand for fuel cells comprising a low profile gas diffusingelectrode.

When the catalyst substance, which may typically be platinum, containedin the gas diffusing electrode is mixed with the other ingredients in apowdery or granular state, the area in which it is brought into contactwith protons (H⁺s) can be insufficient. In other words, the electrodereaction can be insufficient.

SUMMARY OF THE INVENTION

The present invention provides a gas diffusing electrode body that canbe down-sized while maintaining its power generating performance andother characteristics, a method of manufacturing such an electrode bodyand an electrochemical device comprising such an electrode body.

In an embodiment of the invention, the present invention provides a gasdiffusing electrode body including first layers made of at leastelectro-conductive powder or granule and second layers made of acatalyst substance laid alternately to form a multilayer structure.

In another embodiment of the invention, there is provided a method ofmanufacturing a gas diffusing electrode body by laying alternately firstlayers made of at least electro-conductive powder or granule and secondlayers made of a catalyst substance.

In still another embodiment of the present invention, there is providedan electrochemical device including a first pole, a second pole and anion conducting body sandwiched between the poles, at least the firstpole of the first and second poles being formed by a gas diffusingelectrode body having a multilayer structure produced by layingalternately first layers made of at least electro-conductive powder orgranule and second layers made of a catalyst substance.

According to an embodiment of the present invention, oxygen penetratinginto each and every catalyst substance layers of the gas diffusingelectrode body is efficiently ionized due to the use of a multilayerstructure produced by laying alternately first layers made of at leastelectro-conductive powder or granule and second layers made of acatalyst substance so that the reaction in the related electrode isconducted efficiently as oxygen ions and protons (H⁺s) can contact eachother over a large area in each and every layer to make theelectrochemical device comprising such an electrode body operate highlyeffectively and efficiently in each layer and hence the entire device islow-profiled.

The multilayer structure of a gas diffusing electrode body according toan embodiment of the present invention can be formed relatively easilybecause the layers are laid sequentially one on the other. In otherwords, a gas diffusing electrode body according to an embodiment of thepresent invention can be manufactured with an enhanced level ofreproducibility.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription of the Invention and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic cross sectional view of part of an embodiment ofgas diffusing electrode body according to an embodiment of the presentinvention.

FIG. 2 is a schematic cross sectional view of part of another embodimentof gas diffusing electrode body according to an embodiment of thepresent invention.

FIG. 3 is a schematic cross sectional view of part of still anotherembodiment of gas diffusing electrode body according to an embodiment ofthe present invention.

FIG. 4 is a schematic cross sectional view of part of still anotherembodiment of gas diffusing electrode body according to an embodiment ofthe present invention.

FIG. 5 is a schematic cross sectional view of part of still anotherembodiment of gas diffusing electrode body according to an embodiment ofthe present invention.

FIGS. 6A through 6C are schematic cross sectional views of particles ofelectro-conductive carbon powder or granule that can be used accordingto an embodiment of the present invention.

FIGS. 7A and 7B are schematic illustrations of the structure ofpoly-Fullerene hydroxide, which is a Fullerene derivative that can beused according to an embodiment of the present invention.

FIGS. 8A and 8B are schematic illustrations of Fullerene derivativesthat can be used according to an embodiment of the present invention.

FIG. 9 is a schematic illustration of a fuel cell according to anembodiment of the present invention, showing its configuration.

FIG. 10 is a graph illustrating the output performance of a fuel cellaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally relates to gas diffusing electrodes,electrochemical devices that employ same, and methods of manufacturingsame. A gas diffusing electrode body according to an embodiment of thepresent invention includes a multilayer structure of first layers andsecond layers and both the number of the first layers and that of thesecond layers are not smaller than 2 and not greater than 100. The firstlayers are made of at least carbon powder or granule and the secondlayers are made of catalyst metal. Each of the first layers has athickness between several nanometer and several micrometers, which caninclude about μm in an embodiment, while each of the second layers has athickness between several nanometers and hundreds of several nanometers.Preferably, at least one of the second layers contains platinum. On theother hand, preferably, the first layers contain electro-conductivepowder or granule having an ion conducting coat and at least one of thefirst layers contains catalyst metal. Alternatively, it is preferablethat the first layers contain electro-conductive powder or granulehaving a water-repelling coat and the multilayer structure is formed ona collector body or an underlayer. Preferably, the first layers areformed by at least a method selected from a spin coating method, aprinting method, a spray drying method and a vapor phase film formingmethod, whereas the second layers are formed by a vapor phase filmforming method. Preferably, the multilayer structure is formed on acollector body or an underlayer and at least the first pole or thesecond pole is a gas electrode. Preferably, a gas diffusing electrodebody according to an embodiment of the present invention is applied to afuel cell.

Now, the present invention will be described in greater detail byreferring to the accompanying drawings that illustrate preferredembodiments of the present invention.

FIRST EMBODIMENT

Referring to FIG. 1, this embodiment of gas diffusing electrode body isrealized by forming an electro-conductive carbon powder or granule layer22 for allowing electrons to pass through on a gas-permeable collectorbody (carbon sheet) 11 by application, laying thereon a catalyst layer19 typically made of platinum and adapted to turn working gas such asoxygen into oxygen ions by a vapor phase film forming method (such assputtering) and then repeating the operation of laying anelectro-conductive layer 22 and a catalyst layer 19 to produce a gasdiffusing electrode (catalyst layer) 10 as oxygen pole. While FIG. 1shows a two-layered structure for the purpose of simplicity, theembodiment is a multilayer structure having two or more than two layers,although a two-layered structure may feasibly be used according to anembodiment of the present invention. Although not shown, a hydrogen pole(fuel pole) may also be formed in manner as described above for theoxygen pole 10.

Protons (H⁺s) permeate into the gas diffusing electrode (catalyst layer)10 by way of the ion conducting section (proton conductor film) 5 fromthe side of the opposite electrode or from above in FIG. 1, while air(oxygen) permeates into the electrode 10 by way of the gas permeatingcollector (carbon sheet) 11 is ionized there. Then, protons and oxygenions react (cell reaction) with each other in the electrode 10. As aresult, electrochemical energy is taken out as output and water (H₂O) isproduced there.

Note that the catalyst layer 19 that is a platinum layer formed bysputtering does not need to be a continuous film layer. It may be porousand partially discontinued. If it is a continuous film layer, it canblock protons (H⁺s) and gas such as oxygen trying to pass through it.

Electro-conductive powder or granule, which include carbon powder orgranule or the like, can contain micro-pieces of many different profilesincluding particulate pieces, globular pieces, filament-like pieces andthe like.

FIG. 9 is a schematic illustration of a fuel cell according to anembodiment of the present invention, showing its configuration.Referring to FIG. 9, the fuel cell includes an ion conducting section(proton conductor film) 5 having a proton conducting property, anegative pole (fuel electrode) 16 and a positive pole (oxygen electrode)17, the negative pole (fuel electrode) 16 and the positive pole (oxygenelectrode) 17 being arranged on the opposite surfaces of the ionconducting section 5, although the configuration of a fuel cell will bedescribed in greater detail hereinafter.

As hydrogen is supplied to the negative pole (fuel electrode) 16 andoxygen (air) is supplied to the positive pole (oxygen electrode) 17, acell reaction takes place to give rise to electromotive power. In thecase of so-called direct methanol system, methanol may be supplied tothe negative pole (fuel electrode) 16 as hydrogen source.

The negative pole (fuel electrode) 16 and the positive pole (oxygenelectrode) 17 are formed by molding an electrode material mainlycontaining carbon powder or granule 1.

As shown in FIG. 1, this embodiment has a multilayer structure formed byalternately laying carbon powder or granule layers 11 and catalystlayers 19 that are sputtered platinum layers. A gas diffusing electrodehaving such a configuration provides a large area in which gas and thecatalyst metal can contact each other so as to make an electrode reactto take place efficiently. Such a configuration makes it possible torealize a low profile fuel cell.

The carbon powder or granule that is the primary material of theelectrode is required to be electro-conductive. Therefore, any ofgraphite type various carbon materials, carbon nano-tubes and the likecan be used according to an embodiment of the present invention.Particularly, needle-shaped graphite pieces are preferably be used fromthe viewpoint of achieving a high gas diffusing effect.

Carbon nano-tubes are obtained by arranging a cathode and an anode,which are carbon rods typically made of graphite, in a vacuum reactionchamber with a gap separating them from each other, supplying a DCcurrent to the electrodes to cause an arc discharge to take place in anatmosphere of rare gas such as helium and refining the carbon materialdeposited on the inner surface of the reaction chamber.

When a carbon material produced by the above described method is used aselectrode material, a negative pole (fuel electrode) or a positive pole(oxygen electrode) can be directly formed on a gas permeating collector11. Techniques that can be used forming a pole on a collector 11 includespin coating, spraying, dropping and bar coating.

With a spin coating method, carbon powder or granule is dispersed insolvent, which may typically be water or ethanol, and the solvent isdropped directly on a rotating collector. With a spraying method, carbonpowder or granule is dispersed in solvent, which may typically be wateror ethanol, and the solvent is sprayed onto a collector. With a droppingmethod, carbon powder or granule is dispersed in solvent, which maytypically be water or ethanol, and the solvent is dropped directly on acollector. With any of the above described pole forming method, carbonpowder or granule is deposited on the collector.

Carbon nano-tubes show a filament-like profile with a diameter of about1 nm and a length of about 1 to about 10 μm. Each piece of needle-likegraphite has a diameter of about 0.1 to about 0.5 μm and a length ofabout 1 to about 50 μm. Such oblong pieces are entangled with each otherto easily form a layer without using any binding agent, although abinder may be used whenever necessary. In other words, a binderdispersing solution may be used when applying carbon powder or granuleonto a gas permeating collector (carbon sheet). A sputtering method maybe used when a vapor phase film forming process is involved.

The gas diffusing electrode (catalyst layer) 10 may be made to contain acatalyst substance (platinum). To do this, a film layer of the catalystsubstance may be formed by means of a vapor phase film forming methodsuch as sputtering or a mixture containing particles of the catalystsubstance may be used. For instance, a 1000 nm thick film of thecatalyst substrate (platinum) can be formed on a substrate by applyingDC 1A, 420V to a platinum target having a diameter of 5 inch andcarrying out a sputtering operation for 8 minutes and 8 seconds, whiledriving the substrate to rotate.

In the case of a negative pole (fuel cell) or a positive pole (oxygenelectrode) formed by using a gas diffusing electrode according to theinvention, it is no longer necessary to form it separately asself-standing electrode because it is formed directly on a gas diffusingcollector (carbon sheet) typically by spin coating. Therefore, it is notrequired to show a mechanical strength that can resist the risk ofdamages during various processes. In other words, the gas diffusingelectrode can be made to be as thin as about 10 μm or less, typicallybetween about 2 and about 4 μm. However, it may alternatively beprepared as self-standing electrode. Conventional similar structures aresingle layer structures or multilayer structures comprising identicallayers having a thickness of about 50 μm.

Materials that can be used for the proton conducting body of a fuel cellaccording to an embodiment of the present invention include notlimitatively proton (hydrogen ion) conducting polymer materials, such asperfluorosulfonic acid resins (e.g., Nafion®: tradename, available fromDu Pont), polyhydrated polymolybdenum acids, such as H₃Mo₁₂PO₄₀.29H₂O,polyhydrated oxides, such as Sb₂O₅.5.4 H₂O, substances obtained byintroducing proton-dissociating groups into various carbon materialsincluding Fullerene and mixtures of a compound mainly based on siliconoxide and Brönsted acid, a polymer having side chains of sulfonic groupsand/or the like.

Polymer materials such as perfluorosulfonic acid resins, polyhydratedpolymolybdenum acids and polyhydrated oxides show a high protonconductivity at or near room temperature when placed in a wet condition.Take perfluorosulfonic acid resin for example. Protons electrolyticallydissociated from sulfonic acid groups of the resin are bonded tomoisture that is taken up to a large extent into a polymer matrix(hydrogen bond) to produce protonated water, or oxonium ions (H₃O⁺).Since protons in the form of oxonium ions can smoothly move through apolymer matrix, the latter shows a considerably high proton conductioneffect at room temperature. A proton conductor having a conductionmechanism totally different from such a material may alternatively beused. Proton conductors of the latter type include composite metaloxides having a perovskite structure such as SrCeO₃ doped with ytterbium(Yb). It has been found that composite metal oxides having a perovskitestructure shows proton conductivity without using water as proton movingmedium. It is believed that protons are conducted through oxygen ionsthat form the skeleton of the perovskite structure as they channelthrough by themselves.

As pointed out above, materials that can be used for the protonconducting body of a fuel cell according to an embodiment of the presentinvention include substances obtained by introducing proton-dissociatinggroups into various carbon materials such as Fullerene.Proton-dissociating groups as used herein refer to functional groupsthat can electrolytically dissociate protons, such as —OH, —OSO₃H,—SO₃H, —COOH, —OPO(OH)₂ and/or the like, and the expression of“dissociation of protons (H⁺s)” refers to separation of protons fromfunctional groups due to electrolytic dissociation. Protons move througha proton conductor by way of proton-dissociating groups to make it showion conductivity. While any carbon materials can be used for the purposeof the invention so long as they contain carbon atoms as principalingredient, it is necessary that the material shows an ion conductivitythat is higher than the electron conductivity of the material afterintroducing proton-dissociating groups into it. Specific examples ofsuch carbon materials include carbon clusters that are agglomerates ofcarbon atoms and materials containing tube-shaped carbon (so-calledcarbon nano-tubes).

While various carbon clusters are known, Fullerene, carbon clustershaving an open end at part of the Fullerene structure and those having adiamond structure may suitably be used for the purpose of the invention.

Now, carbon clusters will be described in greater detail below.

A carbon cluster is normally an agglomerate of several to hundreds atomsformed as a result of gathering, or agglomeration. If atoms of anagglomerate are carbon atoms, the agglomerate (mass) improves the protonconductivity of carbon. At the same time, it shows a sufficient filmstrength and is apt to form a layer, while maintaining the chemicalproperties of carbon.

A cluster containing carbon as principal ingredient is a mass formed asseveral to hundreds of carbon atoms are bound together regardless of theform of carbon-carbon bonds. Such a cluster is not necessarily formed100% by carbon atoms and may contain atoms other than carbon atoms.Thus, a cluster mainly containing carbon atoms is also referred to ascarbon cluster. In a proton conductor containing a carbon materialhaving proton-dissociating groups, protons can easily be dissociatedfrom such proton-dissociating groups even in a dry condition andconducted to a large extent in a broad temperature range (at least fromabout 160° C. to about −40° C.) including room temperature.

As pointed out above, while the proton conductor shows a sufficientproton conductivity in a dry condition, moisture may exist with it.Moisture may enter from the outside.

The number of layers of the multilayer structure of this embodiment ispreferably between 2 and 100. If the number of layers is less than 2,the structure is simply a single layer structure and does not providethe advantages of the present invention because it does not structurallydiffer from the prior art. On the other hand, if the number of layersexceeds 100, protons (H⁺s) and air (oxygen) can mainly contact thesputtered platinum layers 19 on and near the outer periphery of theelectrode to quickly give rise to an electrode reaction in the initialstages of penetration into the gas diffusing electrode (catalyst layer)10 so that protons (H⁺s) and air (oxygen) may hardly get to the insideof the gas diffusing electrode (catalyst layer) 10 and hence an internalpart of the latter may not participate in the electrode reaction andbecomes simply wasted. Each of the carbon powder or granule layersbecomes thin when the number of the layers is increased without changingthe thickness of the electrode. However, the gas permeability of theelectrode is damaged to suppress the cell reaction and reduce the outputlevel if the number of the carbon powder or granule layers and thesputtered platinum (catalyst metal) layers 19 is raised to over 100layers. Additionally, the overall thickness of the electrode may also beincreased. Therefore, the optimal number of layers under the abovedescribed conditions is typically between 5 to 6 for both the layers 22and the layers 19. However, the situation may change if carbon powder orgranule layers can be formed to show a much lower profile without losingtheir functions, if partly. Preferably, a carbon powder or granule layeris combined with a sputtered platinum layer 19. For example, when acarbon powder or granule layer having a thickness of several micrometersis formed by using carbon powder or granule with a particle diameter oftens of several nanometers, the gas diffusing electrode mayadvantageously be formed by using 100 or less than 100 such layers.

In this embodiment, each electro-conductive carbon powder or granulelayer for conducting electrons is made to show a thickness of severalnanometers to several micrometers, while each sputtered platinum layer19 operating as catalyst layer for decomposing working gas such asoxygen or hydrogen into protons and electrons or ionizing oxygen has athickness of several nanometers to hundreds of several nanometers.

The layers formed by using electro-conductive carbon powder or granulemay be replaced by a substance other than a carbon material if thelatter shows electro-conductive and performs predetermined functions.

The outer diameter, the weight, the forming method and the number oflayers of carbon powder or granule to be used for the above describedembodiment may be modified without limitations from the above listedrespective values if they provide the predetermined effects.

The layers of carbon powder or granule of the embodiment are formed by amethod selected from spin coating, printing, spray drying and vaporphase film forming. However, any method other than those listed abovemay be used if it provides the predetermined effects. A thickness thatis out of the above defined range may be selected for each layer if itprovides the predetermined effects.

The sputtered platinum layers 19 of the above described embodiment thatoperate as catalyst for decomposing working gas such as oxygen orhydrogen into protons and electrons may be replaced by some othercatalyst metal if it provides the predetermined effects. In other words,it is not necessary to form the layers 19 by using platinum. Thethickness of each layer may be modified appropriately if it provides thepredetermined effects.

If the predetermined effects are provided, a third layer and/or anunderlayer may be arranged between the gas permeating collector (carbonsheet) and the embodiment of gas diffusing electrode body.

Thus, the above described embodiment of gas diffusing electrode bodycomprises a multilayer structure realized by alternately laying firstlayers at least made of electro-conductive powder or granule and secondlayers made of a catalyst substance so that oxygen penetrating the gasdiffusing electrode body is efficiently ionized by each of the catalystsubstance layers in the gas diffusing electrode body and produced ionsare brought into contact with protons (H⁺s) over an extended area toimprove the efficiency of reaction in the electrode to raise the outputlevel. Thus, the embodiment performs excellently in each of its layersand, therefore, operates highly effectively and efficiently if each ofthe layers and hence the entire embodiment have a reduced thickness.

Since the layers of the multilayer structure of the above describedembodiment of gas diffusing electrode body are laid sequentially one onthe other, the multilayer structure can be formed relatively easily. Inother words, a gas diffusing electrode body according to an embodimentof the present invention can be manufactured with an enhanced level ofreproducibility. In this regard, the present invention provides a gasdiffusing electrode body including carbon powder or granule layers andcatalyst metal layers laid sequentially and alternately one on theother.

Thus, with a method of manufacturing a gas diffusing electrode bodyaccording to an embodiment of the present invention, carbon powder orgranule layers and layers containing catalyst metal are sequentially andalternately laid one on the other.

In another embodiment of the present invention, there is provided a fuelcell including a pair of gas diffusing electrodes arranged oppositelywith a proton conductor film interposed between them, at least one ofthe pair of gas diffusing electrodes comprising carbon powder or granulelayers and catalyst metal layers laid sequentially and alternately oneon the other.

Thus, a gas diffusing electrode according to an embodiment of thepresent invention and including carbon powder or granule layers andcatalyst metal layers laid sequentially and alternately one on the othercan efficiently perform an electrode reaction can be made to show a lowprofile.

A method of manufacturing a gas diffusing electrode according to anembodiment of the present invention can manufacture a gas diffusingelectrode body including carbon powder or granule layers and catalystmetal layers laid sequentially and alternately one on the other withease.

A fuel cell according to an embodiment of the present invention includesa pair of gas diffusing electrodes arranged oppositely with a protonconductor film interposed between them, at least one of the pair of gasdiffusing electrodes comprising carbon powder or granule layers andcatalyst metal layers laid sequentially and alternately one on theother. Such a fuel cell can be down-sized so as to show a low profile.

SECOND EMBODIMENT

FIG. 2 schematically illustrates a second embodiment of gas diffusingelectrode body according to the invention. The second embodiment differsfrom the first embodiment only in that platinum powder 6 having auniform particle diameter is added as catalyst to the layers of carbonpowder or granule. Otherwise, the second embodiment is identical withthe first embodiment. If the predetermined effects are obtained, theplatinum powder 6 that is added as catalyst may be replaced by powder ofsome other catalyst metal. The outer diameter of the platinum powder 6,the content (weight %) of platinum powder 6 in the layers formed bycarbon powder or granule, the mixing method to be used for the secondembodiment may be modified appropriately if the predetermined effectsare obtained. If platinum powder 6 is added to each and every carbonpowder or granule layer 22 of the gas diffusing electrode layer or notcan be determined without restrictions by considering the predeterminedeffects, although it needs to be added to at least one of the carbonpowder or granule layers 22.

Since this embodiment of gas diffusing electrode body contains platinumpowder as catalyst metal, the catalysing effect of the electrode israised to give rise to a high active electrode reaction. Otherwise, thisembodiment provides advantages similar to those of the first embodiment.

THIRD EMBODIMENT

FIG. 3 schematically illustrates the third embodiment of gas diffusingelectrode body according to the invention. The third embodiment differsfrom the first embodiment only in that particles of carbon powder orgranule provided with an H⁺ conducting film coat is further added to thelayers mainly made of carbon powder or granule. Otherwise, the thirdembodiment is identical with the first embodiment.

As shown in FIG. 3, the surface of some of the spherical particles ofcarbon powder or granule 1 is covered by an H⁺ conducting film coat 20of a Fullerene derivative such as Fullerenol.

The profile of the particles of carbon powder or granule of thisembodiment is not limited to spherical as shown in FIG. 3. An H⁺conducting film coat 20 can be formed on particles showing manydifferent profiles.

The thickness of the H⁺ conducting film coats 20 that covers some of theparticles of carbon powder or granule may be that of a single moleculelayer or more and hence will be at least several nanometers. It ispreferably not more than hundreds of several nanometers because theelectro-conductivity of the coated particles of carbon powder or granulewill be adversely affected if the thickness is too great. Preferably,the H⁺ conducting film coats of carbon powder or granule is between 10nanometers and tens of several nanometers. The surfaces of particles ofcarbon powder or granule can be covered by an H⁺ conducting film coat 20typically by dispersing H⁺ conducting resin into solvent, immersingcarbon powder or granule into the solvent and subsequently drying thesolvent.

Part of the surface of each particle of carbon powder or granule that iscoated with an H⁺ conducting film coat may be further coated with acatalyst substance (e.g., platinum). Since particles of carbon powder orgranule covered by an H⁺ conducting film coat can easily agglomerate,such particles of carbon powder or granule 21 covered by an H⁺conducting film coat form continuous chain structures in the gasdiffusing electrode (catalyst layer) 10 as shown in FIG. 3. Therefore,the chain structures formed by particles of carbon powder or granule 21covered by an H⁺ conducting film coat accelerate the flow of proton gas(H⁺) so that the pores (gaps) that allow proton gas (H⁺) to pass throughare maintained to diffuse gas sufficiently.

It is believed that particles of carbon powder or granule provided withan H⁺ conducting film coat operates advantageous for activating cellreactions when the gas diffusing electrode (catalyst layer) 10 containssuch particles of carbon powder or granule covered by an H⁺ conductingfilm coat by about 1 to about 80 weight %, preferably by about 20 toabout 70 weight %. If the weight % of particles of carbon powder orgranule covered by an H⁺ conducting film coat is too low, the flow ofproton gas (H⁺) is reduced in the gas diffusing electrode (catalystlayer) 10 to prevent gas from passing through and consequently reducethe cell reaction.

If, on the other hand, the weight % of particles of carbon powder orgranule covered by an H⁺ conducting film coat is too high, particles ofcarbon powder or granule poorly contact with each other and becomedistributed unevenly to provide only an insufficient electronconductivity to consequently reduce the cell reaction, although protongas (H⁺) may be able to flow easily.

In the case of a negative pole (fuel electrode) or a positive pole(oxygen electrode) formed by using a gas diffusing electrode accordingto the invention, it is no longer necessary to form it separately asself-standing electrode because it is formed directly on a gas diffusingcollector (carbon sheet) typically by spin coating. Therefore, it is notrequired to show a mechanical strength that can resist the risk ofdamages during various processes. In other words, the gas diffusingelectrode can be made to be as thin as about 10 μm or less, typicallybetween about 2 and about 4 μm. However, it may alternatively beprepared as self-standing electrode.

For this embodiment, the content (weight %) of particles of carbonpowder or granule, that of particles of carbon powder or granuleprovided with an H⁺ conducting film coat and the method of mixing themmay be modified freely provided that the predetermined effects areobtained. Additionally, the type of H⁺ conducting material, the methodof making it adhere to particles of carbon powder or granule and thethickness of the adhering film coat may be modified freely provided thatthe predetermined effects are obtained.

Since the layers made of carbon powder or granule of this embodiment ofgas diffusing electrode body contains particles of carbon powder orgranule provided with an H⁺ conducting film coat, proton gas (H⁺)entering from the ion conducting section sufficiently permeate into thegas diffusing electrode through the H⁺ conducting film coats to giverise to a high active electrode reaction.

Otherwise, this embodiment provides advantages similar to those of thefirst embodiment.

FOURTH EMBODIMENT

FIG. 4 schematically illustrates the fourth embodiment of gas diffusingelectrode body according to the invention. The fourth embodiment differsfrom the first embodiment only in that particles of carbon powder orgranule provided with a water-repellent film coat 26 are further addedto the layers mainly made of carbon powder or granule. Otherwise, thefourth embodiment is identical with the first embodiment.

As shown in FIG. 4, the surfaces of some of the spherical particles ofcarbon powder or granule 1 are covered by a water-repellent film coat18. The profile of the particles of carbon powder or granule of thisembodiment is not limited to spherical as shown in FIG. 4. Awater-repellent film coat can be formed on particles showing manydifferent profiles. The thickness of the water-repellent film coat 18that covers some of the particles of carbon powder or granule may bethat of a single molecule layer or more and hence will be at leastseveral nanometers. It is preferably not more than hundreds of severalnanometers because the electro-conductivity of the coated particles ofcarbon powder or granule will be adversely affected if the thickness istoo great. Preferably, the water-repellent film coats covering particlesof carbon powder or granule has a thickness between 10 nanometers andtens of several nanometers.

Part of the surface of each particle of carbon powder or granule that iscovered by a water-repellent film coat may be further coated with acatalyst substance (e.g., platinum).

Since water generated as a result of intra-electrode reaction does notadhere to the surfaces of particles 26 of carbon powder or granulehaving a water-repellent film coat 18 formed thereon, water generatedwithin the electrodes will not remain there excessively as it isrepelled by the film coat but will be discharged from the electrodes tosecure gaps through which oxygen gas permeates. Therefore, the supply ofoxygen gas into the gas diffusing electrode, or the positive pole, wouldnot be blocked. As a result, oxygen gas is supplied to the electrode ata sufficient rate and hence the output of the cell can be maintained toa relatively high level.

The surfaces of particles of carbon powder or granule 1 can be coveredby a water-repellent film coat 18 by dispersing water-repellent resininto a solvent, immersing particles of carbon powder or granule into thesolvent and subsequently drying the particles.

Materials that can be used for forming the water-repellent film coat 18include fluorine-containing compounds such as polyvinylidene fluoride(PvdF), fluorine type polymers (e.g., C₂F₆ polymer) and Teflon(tradename, PTFE available from Du Pont). Techniques that can be usedforming the water-repellent film coat 18 include the dipping method andthe plasma CVD method.

Since particles of carbon powder or granule covered by a water-repellentfilm coat that are used for a gas diffusing electrode repel water, waterwould not adhere to the outer surfaces of carbon powder or granule.Furthermore, since similar particles of carbon powder or granule coveredby a water-repellent film coat can easily agglomerate, such particles ofcarbon powder or granule 26 covered by a water-repellent film coat formcontinuous chain structures in the gas diffusing electrode (catalystlayer) 10 as shown in FIG. 4. Then, the chain structures formed byparticles of carbon powder or granule 26 covered by a water-repellentfilm coat produce walls that prevent generated water (H₂O) frompenetrating and also paths that are used effectively to drain generatedwater so that the pores (gaps) that allow O₂ gas to pass through aremaintained to diffuse gas sufficiently.

It is believed that particles of carbon powder or granule provided witha water-repellent film coat operate advantageously for activating cellreactions when the gas diffusing electrode (catalyst layer) 10 containsparticles of carbon powder or granule covered by a water-repellent filmcoat by about 1 to about 80 weight %, preferably by about 20 to about 70weight %. If the content by weight % of carbon powder or granule whoseparticles are provided with a water-repellent film coat is too small,water generated within the gas diffusing electrode (catalyst layer) 10as a result of a cell reaction can adhere to the surfaces of particlesof carbon powder or granule to a large extent without being dischargedfrom the electrode and remain within the electrode to fill the gapsthrough which gas can otherwise pass. Then, gas is prevented frompermeating and consequently the cell reaction is weakened. If, on theother hand, the content by weight % of carbon powder or granule whoseparticles are provided with a water-repellent film coat is too large,particles of the carbon powder or granule of the electrode cannotcontact freely and become distributed unevenly to reduce the electronconductivity and weaken the cell reaction.

In the case of a negative pole (fuel cell) or a positive pole (oxygenelectrode) formed by using a gas diffusing electrode according to theinvention, it is no longer necessary to form it separately asself-standing electrode because it is formed directly on a gas diffusingcollector (carbon sheet) typically by spin coating. Therefore, it is notrequired to show a mechanical strength that can resist the risk ofdamages during various processes. In other words, the gas diffusingelectrode can be made to be as thin as about 10 μm or less, typicallybetween about 2 and about 4 μm. However, it may alternatively beprepared as self-standing electrode.

For this embodiment, the content of particles of carbon powder orgranule, that of particles of carbon powder or granule provided with awater-repellent film coat and the method of mixing them may be modifiedfreely provided that the predetermined effects are obtained.

Additionally, the type of water-repellent material, the method of makingit adhere to particles of carbon powder or granule and the thickness ofthe adhering film coat may be modified freely provided that thepredetermined effects are obtained.

Electro-conductive powder or granule that can be used for thisembodiment is not limited to carbon. Some other material that showselectro-conductivity and provides the predetermined effects mayalternatively be used.

As pointed out above, this embodiment of gas diffusing electrode isformed by using mainly electro-conductive carbon powder or granule, withwhich particles of electro-conductive powder or granule whose particlesare provided with a water-repellent film coat is further mixed. Thus,water generated in the gas diffusing electrode body is effectivelyrepelled by the water-repellent film coats of the particles of theelectro-conductive carbon powder or granule and drained from the gasdiffusing electrode without adhering to the electro-conductive carbonpowder or granule. As a result, permeation of gas is not hindered bygenerated water and a sufficient gas permeability is secured within thegas diffusing electrode body.

Furthermore, a gas diffusing electrode body according to the inventioncan be formed by mixing electro-conductive carbon powder or granulewhose particles are provided with a water-repellent film coat with theother materials, it can be manufactured relatively easily withoutrequiring a complex process.

Otherwise, this embodiment provides advantages similar to those of thefirst embodiment.

FIFTH EMBODIMENT

FIG. 5 schematically illustrates the fifth embodiment of gas diffusingelectrode body according to the invention. The fifth embodiment differsfrom the first embodiment only in that particles of carbon powder orgranule provided with a water-repellent film coat 26 and those providedwith an H⁺ conducting film coat 21 are further added to the layers 22mainly made of carbon powder or granule. Otherwise, the fifth embodimentis identical with the first embodiment.

Since particles of carbon powder or granule provided with awater-repellent film coat 26 and those provided with an H⁺ conductingfilm coat 21 that are used for the mixture are described in detail byreferring to the third and fourth embodiments, they will not bedescribed here any further.

In this embodiment, particles of carbon powder or granule provided withan H⁺ conducting film coat 21 and those of carbon powder or granuleprovided with a water-repellent film coat 26 similar to those asdescribed above are added to the carbon powder or granule layer 22 andthe mixing ratio (weight %) of each type of carbon particles, the mixingmethod, the thickness of the layer may be modified appropriately if thepredetermined effects are obtained.

Since the layers mainly made of carbon powder or granule of thisembodiment of gas diffusing electrode body contains particles of carbonpowder or granule provided with an H⁺ conducting film coat, proton gas(H⁺) entering from the ion conducting section sufficiently permeate intothe gas diffusing electrode through the H⁺ conducting film coats 20 andefficiently contact the catalyst metal to give rise to a high activeelectrode reaction.

Since this embodiment of gas diffusing electrode body is formed by usingmainly electro-conductive carbon powder or granule, with which particlesof electro-conductive carbon powder or granule 26 whose particles areprovided with a water-repellent film coat 18 is further mixed, watergenerated in the gas diffusing electrode body is effectively repelled bythe water-repellent film coats 18 of the particles of theelectro-conductive carbon powder or granule 26 and drained from the gasdiffusing electrode without adhering to the electro-conductive powder orgranule. As a result, permeation of gas is not hindered by generatedwater and a sufficient gas permeability is secured within the gasdiffusing electrode body.

Furthermore, a gas diffusing electrode body according to the inventioncan be formed by mixing electro-conductive carbon powder or granulewhose particles are provided with a film coat with the other materials,it can be manufactured relatively easily without requiring a complexprocess.

Since this embodiment is realized by using both particles ofelectro-conductive carbon powder or granule 21 covered by a protonconducting film coat and those powder or granule 26 covered by awater-repellent film coat, the effect of improving the protonconductivity and the water-repellent effect are synergistically combinedto provide an active and highly efficient cell reaction.

Otherwise, this embodiment provides advantages similar to those of thefirst embodiment.

FIGS. 6A, 6B and 6C are schematic cross sectional views or particles ofelectro-conductive carbon powder or granule that can be used for thisembodiment.

Since a physical film forming process is typically used for thisembodiment, platinum (catalyst) 2 adheres to the surfaces of theparticles of the obtained electro-conductive carbon powder or granule 1as shown in FIG. 6A. Such electro-conductive powder or gain provides anexcellent catalytic effect with a small amount and also a sufficientcontact area between the catalyst and gas. In other words, it ispossible to obtain a relatively large specific surface area for thecatalyst that participates in a reaction and improve the catalysingability of the catalyst.

For this embodiment, platinum (catalyst) 2 may adhere unevenly to thesurfaces of the particles of electro-conductive carbon powder or granule1 as shown in FIG. 6B. Such an arrangement of platinum 2 can provide anexcellent catalysing effect with a relatively small amount of catalystas in the case of electro-conductive carbon powder or granule whosegranules having a structure as shown in FIG. 6A and also a sufficientcontact area between the catalyst and gas. In other words, it ispossible to obtain a large specific surface area for the catalyst thatparticipates in a reaction and improve the catalysing ability of thecatalyst.

In place of causing platinum (catalyst) 2 to adhere to the surfaces ofparticles of electro-conductive carbon powder or granule and form a coatfilm there by means of a physical film forming process, it is alsopossible to cause an ion conducting substance 3 to adhere to thesurfaces of particles of electro-conductive carbon powder or granule andthen platinum (catalyst) 2 to further adhere to surfaces of particlescarrying the ion conductive substance and form a film coat there bymeans of a physical film forming process as shown in FIG. 6C. Becauseplatinum (catalyst) 2 is caused to adhere to the surfaces of particlesof electro-conductive carbon powder or granule by means of a physicalfilm forming process, it is no longer necessary to subject the work to aheat treatment process for the purpose of improving the crystallinity ofthe catalyst unlike conventional methods. Then, it is possible to causethe catalyst to adhere to the surfaces of particles ofelectro-conductive carbon powder or granule without damaging the ionconducting property of the ion conducting electrode.

With any of the particles of electro-conductive carbon powder or granuleillustrated in FIGS. 6A, 6B and 6C, the catalyst is made to adhere toelectro-conductive carbon powder or granule preferably at a rate ofabout 10 to about 1,000 weight % relative to the electro-conductivecarbon powder or granule. Preferably, a metal having an electronconductivity is used as catalyst. Examples of such metals includeplatinum, ruthenium, vanadium and tungsten. A mixture of any of suchmetals can also be used as catalyst for the purpose of the invention.

While electro-conductive carbon powder or granule 1 is not subjected toany particular limitations so long as it is acid-resistant,electro-conductive and available at low cost, preferable examples ofelectro-conductive carbon powder or gain include powdery carbon and ITO(indium-tin oxide), of which powdery carbon is particularly favorablefor the purpose of the present invention. The average particle size ofcarbon powder to be used for the purpose of the invention is preferablyabout 1 μm or less, more preferably between about 0.005 and about 0.1μm.

Examples of physical film forming methods that can be preferably usedfor causing the catalyst to adhere to the surfaces of particles ofelectro-conductive carbon powder or granule include sputtering, pulselaser deposition (PLD) and vacuum evaporation. The use of sputtering asphysical film forming process is advantageous because it is easy to useand provides a high productivity and a good film forming effect. Thepulse laser deposition method is advantageous for physical film formingprocesses because the latter can be controlled with ease if the methodis used. The film forming effect of the method is also excellent.

PCT Patent Application Laid-Open Publication No. 11-510311 describes amethod of forming a film of a noble metal on a carbon sheet bysputtering. For the above described embodiment of the present invention,on the other hand, platinum that operate as catalyst is made to adhereto the surfaces of particles of electro-conductive carbon powder orgranule. The method of the present invention is more advantageous thanthe method described in PCT Patent Application Laid-Open Publication No.11-510311 because the specific surface are of platinum operating ascatalyst can be made greater and the catalysing ability of the catalystcan be improved with the former method.

Additionally, for this embodiment of the present invention, when causingplatinum that operates as catalyst to adhere to the surfaces ofparticles of electro-conductive carbon powder or granule to form a filmcoat on the surfaces by a physical film forming method, theelectro-conductive carbon powder or granule is preferably subjected tovibrations in order to cause a sufficient amount of catalyst to adhereuniformly to each carbon particle.

While any appropriate mechanism may be used for applying vibrations tothe electro-conductive carbon powder or granule, a preferably mechanismwill be such that platinum that operates as catalyst is made to adhereto the surfaces of particles of electro-conductive carbon powder orgranule by means of a physical film forming process, while an ultrasonicwave is being applied to generate vibrations in the carbon powder orgranule. With this embodiment of the invention, electro-conductivecarbon powder or granule obtained by causing platinum that operates ascatalyst to adhere to the surfaces of the particles thereof can be boundtogether typically by means of resin. Additionally, theelectro-conductive carbon powder or granule is preferably retained on aporous gas permeating collector such as a carbon sheet.

As described above, this embodiment of gas diffusing electrode accordingto an embodiment of the present invention may be substantially made ofelectro-conductive carbon powder or granule whose particles are providedwith a film coat or it may contain electro-conductive carbon powder orgranule and resin for binding the particles of the powder or granule aswell as other ingredients. In the latter case, such other ingredientsmay include a pore forming agent (e.g., CaCO₃) and ion conductors.Additionally, the electro-conductive carbon powder or granule ispreferably retained on a porous gas permeating collector such as acarbon sheet.

Examples of ion conductors that can be used in a gas diffusing electrodeand also in the ion conducting section sandwiched between the first andsecond poles of an electrochemical device according to the inventioninclude Nafion® (tradename, perfluorosulfonic acid resin available fromDu Pont) as well as Fullerene derivatives such as Fullerenol(Fullerene-polyhydrorxide).

Particularly, synthesis of Fullerenol having a chemical structureobtained by adding a plurality of hydroxyl groups as shown in FIGS. 7Aand 7B was firstly reported by Chiang et al. in 1992 (Chiang, L. Y.;Swirczewski, J. W.; Hsu, C. S.; Chowdhury, S. K.; Cameron, S.; Creegan,K., J. Chem. Soc., Chem. Commun., 1992, 1791).

The applicant of the present patent application caused hydroxyl groupsof Fullerenol molecules that were brought close to each other forforming agglomerates as shown in FIG. 8A (◯ denotes a Fullerene moleculein FIG. 8A) to act on each other and found for the first time that theagglomerate shows a high proton conductivity as macro-aggregate and H⁺sare easily dissociated from phenolic hydroxyl groups of Fullerenolmolecules.

For this embodiment of the present invention, Fullerene agglomerateshaving a plurality of —OSO₃H groups may be used as ion conductor otherthan Fullerenol. Poly-Fullerene hydroxide obtained by replacing OHgroups with OSO₃H groups as shown in FIG. 8B was also reported by Chianget al. in 1994 (Chiang, L. Y.; Wang, L. Y.; Swirczewski, J. W.; Soled,S.; Cameron, S., J. Org. Chem., 1994, 59, 3960). Fullerenehydrogensulfate may have only OSO₃H groups or OSO₃H groups and hydroxylgroups at a same time in each molecule.

When Fullerenol and Fullerenol hydrogensulfate are made to produce alarge number of agglomerates, protons coming from a large number ofhydroxyl groups and OSO₃H groups of molecules of these compoundsdirectly participate in the movement of protons so that it is notnecessary to take in hydrogen and protons from the atmosphere, steammolecules and other sources to secure a desired level of protonconductivity for the bulk. In other words, it is not necessary toexternally supply moisture from outer air. Therefore, the atmospheredoes not need to be subjected to restrictions. Thus, a gas diffusingelectrode according to the invention can be used continuously even in adry atmosphere.

Fullerene from which such molecules as listed above are formed shows anexcellent proton conductivity because it has a high electrophilicity,which, it is believed, participate in promoting the electrolyticdissociation of hydrogen ions not only from highly acidic OSO₃H groupsbut also from hydroxyl groups. Additionally, because a considerablenumber of hydroxyl groups and OSO₃H groups can be introduced into eachmolecule, the numerical density of proton conductors that participate inproton conduction is very high to realize an effective conductivitylevel.

Since Fullerenol and Fullerenol hydrogensulfate are constituted mostlyby carbon atoms of Fullerene, they are lightweight and chemically hardlymodified. They do not contain environment-contaminating substances.Furthermore, the Fullerene manufacturing cost is falling rapidly. Thus,it will be safe to say that Fullerene is a desirable carbon materialfrom the viewpoint of resource, environment and economy if compared withother materials. Furthermore, it is possible to make Fullerene moleculeshave any of —COOH, —SO₃H and —OPO(OH)₂ in addition to —OH and —OSO₃H.

When synthetically producing Fullerenol and other materials that can beused for the purpose of the present invention, desired groups areintroduced to carbon atoms of Fullerene molecules by using knownprocesses such as acid treatment and hydrolysis in combination. When aFullerene derivative is used as ion conductor for the ion conductingsection, it is preferably that the ion conductor is substantially madeof only the Fullerene derivative or bound together by means of a bindingagent.

This embodiment of gas diffusing electrode according to the inventioncan suitably be used for various electrochemical devices having a firstpole, a second pole and an ion conductor sandwiched by the two poles asbasic structure. More specifically, a gas diffusing electrode accordingto the invention can be used at least for the first pole of the firstand second poles.

To be more accurate, a gas diffusing electrode according to theinvention can suitably be used for an electrochemical device havingfirst and second poles, at least one of which is a gas electrode.

Now, a fuel cell realized by using a gas diffusing electrode accordingto the invention will be described specifically and briefly by referringto FIG. 9.

The illustrated fuel cell includes a negative pole 16 and a positivepole 17 that are disposed vis-a-vis and formed by using respective gasdiffusing electrode 10 and a ion conducting section 5 sandwiched betweenthe two poles. The negative pole 16 is a fuel electrode or hydrogenelectrode, whereas the positive pole 17 is an oxygen electrode, the ionconducting section 5 being realized as a proton conductor section.Terminals 15, 14 are drawn respectively from the negative pole 16 andthe positive pole 17 and connected to a load 4 that is an externalcircuit.

When the fuel cell is in use, hydrogen is supplied from an inlet port(not shown) and discharged from an outlet port (not shown) at thenegative pole 16. The outlet port may be omitted.

While fuel gas (H₂) is driven to pass through H₂ flow path 12, hydrogenis diffused into the negative pole, where protons (H⁺s) are generated.Then, the generated protons (H⁺s) move to the positive pole 17 alongwith the protons generated in the ion conducting section (protonconductor section) and react with the oxygen (air) that is supplied toO₂ flow path 13 and directed toward its outlet port (not shown) so thatconsequently desired electromotive force can be taken out.

Although not referred to in the above description of the arrangement,the hydrogen supply source stores a hydrogen occluding alloy and/or ahydrogen occluding carbon material. The hydrogen occluding carbonmaterial may be made to occlude hydrogen in advance before it is storedin the hydrogen supply source.

Since the fuel cell includes a first pole and/or a second pole formed byusing this embodiment of gas diffusing electrode according to anembodiment of the present invention, it provides an excellent catalysingeffect and can secure a sufficient contact areas for the catalyst andgas (H₂ or the like) so that the catalyst participating in the reactioncan have a large specific surface area to improve its catalysing abilityand ensure a good output performance. Additionally, hydrogen ions aredissociated in the negative pole 16 and also in the ion conductingsection 5 and move toward the positive pole 17 so that the fuel cellshows a high ion conductivity even in a dry condition. Therefore, a fuelcell according to an embodiment of the present invention does notrequire the use of a moisturizing device so that the entire system canbe simplified and lightweight. Thus, the electrodes show an improvedperformance particularly in terms of current density and outputcharacteristics.

The film-shaped ion conducting body that is formed bycompression-molding a Fullerene derivative and sandwiched between thefirst and second poles may be replaced by an ion conducting section 5formed by binding a Fullerene derivative together by means of a bindingagent. Then, the ion conducting section will show a satisfactorystrength because a binding agent is used for binding the Fullerenederivative together. Polymer materials that can be used as binding agentfor the purpose of the invention include known polymers having a filmforming property, of which one or more than one will be used. Thepolymer content of the ion conducting section is normally about 20weight % or less because the hydrogen ion conductivity of the ionconducting section can be reduced if the polymer content exceeds 20weight %. Since the ion conducting section arranged in a manner asdescribed above also contains a Fullerene derivative as ion conductor,it shows a satisfactory hydrogen ion conductivity just like an ionconducting section containing only a Fullerene derivative.Advantageously, the ion conducting section arranged in a manner asdescribed above is provided with an excellent film forming property dueto the polymer material added thereto unlike an ion conducting sectioncontaining only a Fullerene derivative. Therefore, it can be used as aflexible ion conducting thin film that shows an enhanced strength and agood gas permeation preventing ability if compared with a compressed andmolded product of powder of a Fullerene derivative (having a filmthickness of not greater than 300 μm).

There are no particularly limitations to polymer materials that can beused according to an embodiment of the present invention so long as theydo not significantly degrade the hydrogen ion conductivity of the ionconducting section (due to the reaction with the Fullerene derivative)and shows a film forming ability. Normally, a polymer showing noelectron conductivity and having an excellent stability will beselected. Examples of such polymers include polyfluoroethylene andpolyvinylalcohol, which are preferable polymer materials for the purposeof the invention because of the following reasons.Polytetrafluoroethylene is preferably used according to an embodiment ofthe present invention because a thin film having an excellent strengthcan be formed with ease if it is contained only to a slight extent whencompared with other polymer materials. The polytetrafluoroethylenecontent of the ion conducting thin film is not greater than about 3weight %, preferably between about 0.5 and about 1.5 weight %, which isvery low, so that the thin film can be made to show a small filmthickness between about 1 and about 100 μm. Polyvinylalcohol ispreferably used for the purpose of the present invention because it canform an ion conducting thin film showing an excellent gas permeationpreventing ability. The polyvinylalcohol content of the ion conductingthin film is preferably between about 5 and about 15 weight %.

The film forming performance can be adversely affected if the content ofpolyfluoroethylene or polyvinylalcohol falls short of the above definedlower limit.

Various known film forming processes including compression molding andextrusion molding can be used for forming a thin film for the ionconducting section of this embodiment where a Fullerene derivative isbound together by means of a binding agent.

There are no specific limitations imposed on the ion conducting bodysandwiched between a pair of gas diffusing electrodes in anelectrochemical device according to the invention so long as it shows agood ion conductivity, a (hydrogen) ion conductivity in particular.Examples of materials that can be used for the purpose of the inventioninclude Fullerene hydroxide, Fullerene hydrogensulfate and Nafion®.Additionally, a binding agent can be used as water-repellent resin for agas diffusing electrode according to the invention.

Now, the present invention will be described further by way of exampleswithout limitation to the scope of the present invention.

EXAMPLE 1

In this example, layers of carbon powder or granule 22 and (sputtered)platinum layers 19 are formed alternately on a gas permeating collectors(carbon sheet) to produce a gas diffusing electrode having aconfiguration as shown in FIG. 1. Then, a fuel cell was prepared byusing the electrode.

For each layer of carbon powder or granule 22, paint obtained bydispersing 0.6 g of ordinary carbon powder or granule 1 (particlediameter: 30 to 40 nm) into 40 g of solvent NMP (N-methylpyrrolidone)was made to drop for 5 seconds by driving a spin coater to rotate at arate of 500 rmp and subsequently for 30 seconds also by driving the spincoater to rotate at a rate of 1,000 rmp. Then, the dropped paint washeated and dried at 120° C.

On the other hand, a sputtered platinum (catalyst substance) film layer19 was formed to a thickness of 20 nm by using a platinum target havinga diameter of 5 inches and applying DC 1A, 420V, while driving asubstrate to rotate for sputtering for 8 minutes and 8 seconds.

5 layers of 300 nm thick carbon powder or granule 22 were formed by spincoating to make the total thickness equal to 1,500 nm. Similarly 5layers of 20 nm thick (sputtered) platinum layers 19 were formed bysputtering to make the total thickness equal to 100 nm. To be moreaccurate, they were formed alternately on a gas permeating collector(carbon sheet) to produce a 1.6 μl thick gas diffusing electrode.

Then, the obtained gas diffusing electrode layer was placed between anion exchange film (proton conducting section) made of nylon and acollector electrode and hydrogen gas and oxygen gas were introduced intothe fuel cell. The relationship between the number of layers of carbonpowder or granule (to be referred to simply as number of layershereinafter) and the output voltage of the fuel cell was observed.

EXAMPLE 2

A gas diffusing electrode was prepared as in Example 1 except thatplatinum powder 6 with a particle diameter of 2 to 3 nm was added to thelayer of carbon powder or granule 22 by 20 weight %.

The obtained gas diffusing electrode layer was placed between an ionexchange film (proton conducting section) made of nylon and a collectorelectrode and hydrogen gas and oxygen gas were introduced into the fuelcell. The relationship between the number of layers of carbon powder orgranule and the output voltage of the fuel cell was observed. The resultwill be described in detail hereinafter.

EXAMPLE 3

A gas diffusing electrode was prepared as in Example 1 except thatparticles of carbon powder or granule 21 provided with an H⁺ conductingfilm coat of Fullerenol were added to the layers of carbon powder orgranule 22 as shown in FIG. 3.

The thickness of the H⁺ conducting film coats 20 formed on particles ofcarbon powder or granule 21 was within a range between 10 nanometers andtens of several nanometers. The compounding ratio of the particles ofcarbon powder or granule 21 provided with an H⁺ conducting film coat andthose of carbon powder or granule 1 without a film coat was 1:1 byweight.

The weight of the H⁺ conducting film coats took 30% of the total weightof the particles of carbon powder or granule 21 provided with an H⁺conducting film coat of an H⁺ conducting Fullerene derivative. Particlesof carbon powder or granule 21 provided with an H⁺ conducting film coatand those of carbon powder or granule 1 without any H⁺ conducting filmcoat were mixed.

The obtained gas diffusing electrode was mounted in a fuel cell as inExample 1 and the output of the cell was observed.

EXAMPLE 4

A gas diffusing electrode was prepared as in Example 1 except thatparticles of carbon powder or granule 26 provided with a water-repellentfilm coat 18 were added to the layers of carbon powder or granule 22 asshown in FIG. 4.

The thickness of the water-repellent coats formed on particles of carbonpowder or granule was within a range between 10 nanometers and tens ofseveral nanometers. The compounding ratio of the particles of carbonpowder or granule provided with a water-repellent film coat and those ofcarbon powder or granule without a film coat was 1:1 by weight.

The weight of the water-repellent coats took 30% of the total weight ofthe particles of carbon powder or granule provided with awater-repellent film coat. Particles of carbon powder or granuleprovided with a water-repellent film coat and those of carbon powder orgranule without any water-repellent film coat were mixed.

The obtained gas diffusing electrode was mounted in a fuel cell as inExample 1 and the output of the cell was observed.

EXAMPLE 5

A gas diffusing electrode was prepared as in Example 1 except thatparticles of carbon powder or granule 21 provided with an H⁺ conductingfilm coat and those of carbon powder or granule 26 provided with awater-repellent film coat were added to the layers of carbon powder orgranule 22 as shown in FIG. 5.

Both the thickness of the water-repellent film coats formed on particlesof carbon powder or granule 26 and that of the H⁺ conducting film coatsformed on particles of carbon powder or granule 21 were within a rangebetween 10 nanometers and tens of several nanometers. The compoundingratio of the particles of carbon powder or granule 26 provided with anH⁺ conducting film coat, those of carbon powder or granule 21 providedwith a water-repellent film coat and those of carbon powder or granulewithout a film coat was 0.5:0.5:1 by weight.

Water-repellent film coats were formed on particles of carbon powder orgranule 26 by immersing carbon powder or granule into a Teflon solutionand then drying it. The weight of the water-repellent coats of Teflontook 30% of the total weight of the particles of carbon powder orgranule provided with a water-repellent film coat.

H⁺ conducting film coats were formed on particles of carbon powder orgranule 21 by immersing carbon powder or granule into a tetrahydrofuranesolution of a Fullerene derivative and then drying it. The weight of theH⁺ conducting film coats of the Fullerene derivative took 30% of thetotal weight of the particles of carbon powder or granule provided withan H⁺ conducting film coat.

Particles of carbon powder or granule provided with a water-repellentfilm coat, those of carbon powder or granule provided with an H⁺conducting film coat and those of carbon powder or granule without anyfilm coat were mixed.

The obtained gas diffusing electrode was mounted in a fuel cell as inExample 1 and the output of the cell was observed.

COMPARATIVE EXAMPLE 1

In this comparative example, a fuel cell was prepared by using aconventional gas diffusing electrode prepared by spraying a dispersedsolution containing platinum particles having an average particlediameter of 100 nanometers onto a 2 μm thick gas permeating collector(carbon sheet).

The content of platinum particles of the dispersed solution was 20weight % and the volume of the dispersed solution was 200 μ liter. Thedispersed solution infiltrated into the carbon sheet.

The obtained gas diffusing electrode layer was placed between an ionexchange film (proton conducting section) and a collector electrode andhydrogen gas and oxygen gas were introduced into the fuel cell. Therelationship between the number of layers of carbon powder or granuleand the output voltage of the fuel cell was observed.

COMPARATIVE EXAMPLE 2

In this comparative example, a fuel cell was prepared by using aconventional gas diffusing electrode prepared by spraying a dispersedsolution same as the one used in Comparative Example 1 onto a 50 μmthick gas permeating collector (carbon sheet). Otherwise the procedureof Comparative Example 1 was followed.

The obtained gas diffusing electrode was placed between an ion exchangefilm (proton conducting section) and a collector electrode and hydrogengas and oxygen gas were introduced into the fuel cell. The relationshipbetween the number of layers of carbon powder or granule and the outputvoltage of the fuel cell was observed.

COMPARATIVE EXAMPLE 3

A fuel cell was prepared as in Comparative Example 1 except that a 1.5μm thick layer of carbon powder or granule was arranged between the gaspermeating collector (carbon sheet) and the dispersed solution ofplatinum particles.

The results obtained in the above examples and comparative examples aresummarily listed below.

TABLE 1 electrode thickness electrode thickness specimen (μm) (μm)Example 1 1.6 0.6 Example 2 1.6 0.7 Example 3 1.6 0.7 Example 4 1.6 0.6Example 5 1.6 0.7 Comparative Example 1 2 0.4 Comparative Example 2 500.6 Comparative Example 3 3.5 0.4

As seen from above, the electrode of Example 1 had a thickness of 1.6 μmand the output voltage was 0.6 V. The cell operated well. The results ofExamples 2 through 5 were similar to those of Example 1.

The electrode of Comparative Example 1 showed a thickness of 2 μm and alow output voltage of 0.4V. The electrode of Comparative Example 2 wasas thick as 50 μm, although it produced a high output voltage of 0.6V.The outcome of Comparative Example 3 was similar to that of ComparativeExample 1.

In Comparative Example 1, since platinum particles were dispersedsporadically, they operated poorly as catalyst and the output voltagewas insufficient. While the output voltage of the specimen ofComparative Example 2 was sufficient, that of the specimen ofComparative Example 3 was similar to the value obtained in ComparativeExample 1. The specimen of Comparative Example 2 was not suited for alow-profiled electrode, although the output voltage was sufficient.

The gas diffusing electrode of Example 1 showed a thickness of 1.6 μm,which was about one thirties of the thickness of the electrode ofComparative Example 2 but the output voltage (0.6 V) was as high as thatof the specimen of Comparative Example 2. The output voltages of thespecimens of Examples 2 through 5 were similar to that of the specimenof Example 1.

From the above results, it was proved that a fuel cell realized by usinga gas diffusing electrode according to an embodiment of the presentinvention can be made to show a reduced profile if compared with fuelcells comprising conventional electrodes, while maintaining a relativelyhigh output voltage.

FIG. 10 is a graph showing the relationship between the output voltageand the number of layers of the gas diffusing electrode of the fuel cellof Example 1.

As seen from FIG. 10, the output voltage is maintained to about 0.6 Vwhen the number of layer is not less than 2 and not more than 100 butgradually falls as the number of layers exceeds 100 to get to 0.54 Vwhen the number of layers is raised to 150 and to 0.44 V when the numberof layers becomes equal to 200.

Thus, the number of layers is preferably between 2 and 150 (less than120 if possible) and more preferably between 2 and 100 for maintainingthe output voltage to a relatively high level.

While the present invention is described above by way of preferredembodiments, they may be modified or altered appropriately withoutdeparting from the spirit and scope of the present invention.

For instance, the above described electrochemical device that is adaptedto give rise to a cell reaction of decomposing H₂ may also be applied toproducing H₂ or H₂O by reversing the chemical process.

So long as the predetermined effects are achieved, the number of layersof carbon powder or granule and that of sputtered platinum layers may bedifferentiated in a gas diffusing electrode. Similarly, layers havingdifferent thicknesses and different compositions that are produced bydifferent forming methods may be combined to form a multilayerstructure.

According to an embodiment of the present invention, there is provided agas diffusing electrode body including first layers made of at leastelectro-conductive powder or granule and second layers made of acatalyst substance laid alternately to form a multilayer structure. Withsuch an arrangement, oxygen penetrating the gas diffusing electrode bodyis efficiently ionized in each of the catalyst layers to expand the areawhere ions contact protons (H⁺s) and efficiently conduct reactions inthe electrode so that a cell realized by using an electrode according tothe invention performs excellent to produce a high output voltage. Sincethe above effects are achieved in each layer, the electrode operatehighly effectively and efficiently if it is made very thin to reduce theoverall thickness of the electrode.

The multilayer structure of a gas diffusing electrode body according toan embodiment of the present invention can be formed relatively easilyby laying the layers one on the other. Thus, a gas diffusing electrodebody according to an embodiment of the present invention can bemanufactured with an enhanced level of reproducibility.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present invention andwithout diminishing its intended advantages. It is therefore intendedthat such changes and modifications be covered by the appended claims.

The invention claimed is:
 1. A method of manufacturing a gas diffusingelectrode body, the method comprising: forming an alternating layerincluding a first layer and a second layer, the first layer including anelectro-conductive carbon powder or granule thereof, and anelectro-conductive carbon powder or granule thereof having awater-repellant film coat, and the second layer consisting essentiallyof one or more catalyst metals; and forming a multilayered structureincluding at least two of said alternating layers, wherein thealternating layers are laid sequentially, where a weight ratio of theelectro-conductive carbon power or granule thereof to theelectro-conductive carbon power or granule thereof having thewater-repellant film coat is about 1:1.
 2. A method of manufacturing agas diffusing electrode body, the method comprising: forming analternating layer including a first layer and a second layer, the firstlayer including an electro-conductive carbon powder or granule thereof,and an electro-conductive carbon powder or granule thereof having awater-repellant film coat, and the second layer consisting essentiallyof one or more catalyst metals; and forming a multilayered structureincluding at least two of said alternating layers, wherein thealternating layers are laid sequentially, wherein the first layerfurther includes an electro-conductive carbon powder or granule thereofhaving an ion conducting film.
 3. A method of manufacturing a gasdiffusing electrode body, the method comprising: forming an alternatinglayer including a first layer and a second layer, the first layerincluding an electro-conductive carbon powder or granule thereof, and anelectro-conductive carbon powder or granule thereof having awater-repellant film coat, and the second layer consisting essentiallyof one or more catalyst metals; and forming a multilayered structureincluding at least two of said alternating layers, wherein thealternating layers are laid sequentially, wherein the electro-conductivecarbon powder of granule thereof having the catalyst metal is made bydepositing a catalyst metal on the electro-conductive carbon powder orgranule thereof and with vibrating the electro-conductive carbon powderor granule thereof, wherein the first layer further includes anelectro-conductive carbon powder or granule thereof having a catalystmetal.